PFBC plants have been described in a great number of publications and patent specifications. One example of such a description is given in EP 0124842 entitled "Power plant with a fluidized bed combustion chamber" and corresponding to U.S. Pat. No. 4,530,207. A so-called PFBC plant includes a compressor unit and a turbine unit in which a low-pressure compressor and a low-pressure turbine are interconnected via a first shaft and a high-pressure compressor and a high-pressure turbine are interconnected via a second shaft.
In a PFBC plant there is a relatively clear-cut relationship between the bed level of the combustor and the load. To be able to maintain this relationship and to obtain optimum operation, a large number of more or less connected systems are required. To cover the fuel consumed during the combustion, the proper quantity of fuel must be continuously supplied to the combustor. This is done with the aid of a fuel supply control. The combustion of supplied mass for optimum use thereof and for keeping the waste gases within permissible limits requires at all times the correct amount of air flow to the pressure vessel and to the bed of the PFBC plant. According to the above, the present invention comprises a control system which satisfies these requirements.
Upon a change of load, in addition to both the fuel supply and the air flow being changed, also the bed level must be adapted to the new load. The bed level may be adjusted in several ways, among other things by feeding bed material from the bed either out into a storage vessel located outside the bed, or from the storage vessel into the bed. This method is described, inter among other things in the above-mentioned EP specification.
The system which supplies the combustor with compressed air comprises a two-shaft gas turbine in which, as mentioned above, the low-pressure compressor (LC) and the low-pressure turbine (LT) are interconnected by a fixed shaft and the high-pressure compressor (HC) and the high-pressure turbine (HT) are interconnected by a fixed shaft. The high-pressure unit drives a generator via a star gear.
In addition to the system mentioned, PFBC plants comprise means for control of bed temperature, feedwater and the like, which are all in different ways connected in dependence on the prevailing load and load changes. Most of these systems have limitations from the point of view of process engineering. For the air flow control these comprise use the difference pressure between the pressure vessel and the bed vessel as well as the pressure ratio between the inlets and outlets of compressors, the minimum and maximum speed of the low-pressure shaft, the rate of change of several of the controlled quantities and the like which become active and influence each other.
The passage of the air through the compressors and up to the fluidized bed as well as the passage of the corresponding waste gases through the turbines and out into the free environment are part of the prior art and will therefore only be briefly described here. Air is admitted into the low-pressure compressor and is passed on via an intermediate cooler to the high-pressure compressor. From there the air is passed into the pressure vessel which thereby becomes pressurized with a certain overpressure in relation to the combustor. The waste gases which are generated during the combustion are now passed from the combustor via cyclones to the high-pressure turbine and from there on to the low-pressure turbine from where the gases are released, possibly after cleaning and cooling, into the free environment.
The air and gas passage through the compressor and turbine units, as described above, need not, of course, be associated with a fluidized bed plant. SE 8602003-9 corresponding to U.S. Pat. No. 4,893,466 entitled "A method for operation of a turbine unit" describes how such a turbine unit may be used together with other types of combustors for, for example, operation of an electric generator connected to the common shaft for the high-pressure compressor and the high-pressure turbine. From the design of this plant it can be deduced that the device for executing control of the power of the generator consists of a guide vane ring, connected to the inlet of the low-pressure turbine, with adjustable guide vanes through which the gas flow from the high-pressure turbine and to the low-pressure turbine has to pass.
Otherwise, it is of interest to note that the above SE patent only mentions devices for air supply under pressure to the combustor and does not disclose any form of the air flow control. As far as it is known, no individual control of the air flow in known PFBC plants exists at present.